The present invention relates to methods of making absorbable polylactone copolymers suitable for use in implantable medical devices, which methods include the use of both mono- and di-functional polymerization initiators, otherwise known as molecular weight control agents, to polymers prepared by such methods and to medical devices prepared from such polymers.
Homopolymers and copolymers of p-dioxanone (PDO) are known for use in the medical device and pharmaceutical fields due to their low toxicity, softness and flexibility. Poly(p-dioxanone) (PDS) homopolymer in particular has been suggested as an absorbable polymer for use in synthetic surgical devices. By the early 1980""s the PDS homopolymer was used by surgeons in the form of a monofilament surgical suture. Since that time, many p-dioxanone copolymers have been described for use in such devices. Surgical monofilament sutures based on a copolymer prepared from trimethylene carbonate (TMC), glycolide (GLY) and p-dioxanone (PDO) monomer currently are available for use. PDO based polymeric materials also can be injection molded into a number of non-filamentous surgical devices such as surgical clips and fasteners for use in, e.g. meniscal repair. These surgical articles take full advantage of the general toughness exhibited by this family of homopolymers and copolymers known heretofore.
Segmental block copolymers composed of p-dioxanone and glycolide (at a molar ratio of PDO:GLY of approximately 90:10) were thought to be polymers potentially suitable for use as a xe2x80x9csoftxe2x80x9d monofilament suture having a break strength retention (BSR) profile similar to Vicryl(copyright) sutures, available from Ethicon, Inc. However, these copolymers were found to crystallize relatively slowly upon cooling from the melt, making the fiber manufacturing processes more difficult and challenging, and thus hampering the conversion of such polymers to xe2x80x9csoftxe2x80x9d monofilament sutures. It would be advantageous, then, to determine polymerization process parameters necessary to produce such polymers having properties suitable for conversion to xe2x80x9csoftxe2x80x9d monofilament sutures, as well as other implantable medical devices. The present invention provides such processes, polymers made by such processes and having unique properties, and medical devices made from such polymers.
The present invention is directed to a polymerization process for making absorbable polylactone copolymers, wherein from about 2 to about 80 mole percent of a first lactone monomer comprising glycolide and about 98 to about 20 mole percent of a second lactone monomer other than glycolide are contacted with a monofunctional polymerization initiator and a di-functional polymerization initiator, at a molar ratio of monofunctional to di-functional initiator ranging from 10:90 to 90:10, and the glycolide and second lactone monomers are in the presence of the monofunctional and di-functional polymerization initiators under conditions effective to copolymerize the monomers, thereby providing the absorbable polylactone copolymer. When medical devices are manufactured from certain polymers prepared by such processes, the rate of crystallization during formation of the device is at least about 2 times faster, and preferably greater than about 10 times faster, than the rate of crystallization when a polylactone polymer made by a substantially similar or the same polymerization process, but utilizing either the monofunctional or the di-functional polymerization initiator alone, is used to form the device. Thus, the present invention provides increased crystallization rates as compared to conventional processing, as taken under the same or similar measurement conditions or techniques. The invention also is directed to absorbable polylactone copolymers prepared by processes of the present invention and to medical devices comprising such copolymers.